Perfectly caramelized onions are the heart of this dish, bringing a sweetness that balances the savory broth and a touch of sherry. Topped with golden, melted cheese over crisp French bread, each bite is irresistibly satisfying.
In a large stockpot (approximately 8 quarts), melt the butter with olive oil over medium heat. Add the sliced onions to the pot and stir continuously until the onions become soft and translucent. Avoid browning the onions during this step to maintain their sweetness.
Pour in the beef broth, add the dry sherry and thyme, and season the mixture with salt and pepper. Allow the soup to simmer for 30 minutes, enabling the flavors to meld.
Preheat the broiler of your oven.
Once the soup has simmered, ladle it into oven-safe serving bowls. Place a slice of French bread on top of each bowl of soup; if desired, break the bread into pieces. On each bread slice, layer one slice of provolone cheese, half of a diced Swiss cheese slice, and sprinkle with a tablespoon of grated Parmesan.
Arrange the bowls on a baking sheet and place them under the preheated broiler. Broil until the cheese is melted, bubbly, and slightly browned, about 2-3 minutes.